This application is based on patent application Nos. 11-47740 and 11-47741 filed in Japan, the contents of which are hereby incorporated by references.
This invention relates to a switchback device for use in image forming apparatus such as copiers, laser printers, and facsimile machines, and particularly to a switchback device for use in image forming apparatus in the technical field of electrophotography.
FIG. 9 is a diagram of a switchback device of prior art for use in an image forming apparatus. Hereinafter, a conventional switchback device is described referring to FIG. 9. The conventional switchback device 7 comprises: a tray 73; a sheet transporting unit 74 including a switchback roller 741, a switchback roller driver (not shown), a movable arm 742, an arm shaft 743, a solenoid 744, an arm bias member 745, and a spring 746; and a sheet slapping unit 75 including a guide member 751, a guide shaft 752, a solenoid 753, a guide bias member 754, and a spring 755.
An operation of the switchback device 7 is described. When the switchback device 7 is in a stand-by state, a rotation of the switchback roller 741 is suspended. In this state, the switchback roller 741 is set to a retracted state above the tray 73, and the guide member 751 is also set to a retracted state (shown by the broken line in FIG. 9).
When a sheet P is transported from an upstream-located path 24 with respect to a sheet transport direction of the image forming apparatus inside the switchback device 7, an inlet guide roller pair 241, 242 is driven to transport the sheet P in a sheet entering direction shown by the solid arrow xe2x86x92 on the tray 73. When the lead end of the sheet P in the entering direction is about to slide under the switchback roller 741, the switchback roller 741 is pivoted downward to come into contact with the sheet P on the tray 73 and guides the sheet P on the tray 73 further in the entering direction (rightward direction in FIG. 9) by transmitting a counterclockwise rotational driving force of the switchback roller 741 in FIG. 9 to the sheet P which is in contact with the switchback roller 741. When the entirety of the sheet P is guided inside the switchback device 7, the solenoid 753 is turned on, and the guide bias member 754 is changed from the broken-line state in FIG. 9 to the solid-line state shown in FIG. 9 against the spring force of the spring 755. When the guide bias member 754 is set to the solid-line state, the guide member 751 is pivoted downward about the axis of the guide shaft 752 due to the weight thereof, thereby slapping a portion of the sheet P, which is certain distance away from a tail end thereof, while the sheet P is guided in the entering direction onto the tray 73. In this way, the sheet P lands on the tray 73. Then the guide member 751 is pivoted upward to resume its broken line state.
Subsequently, the switchback roller 741 is rotated in the reverse direction (clockwise direction in FIG. 9) to transport the sheet P in exiting direction on the tray 73 (shown by the arrow ← in FIG. 9) toward an outlet guide roller pair 251, 252 disposed on a downstream-located path 25 with respect to the sheet transport direction of the image forming apparatus. Thus, the sheet P exits the switchback device 7 by the outlet guide roller pair 251, 252.
In the above conventional switchback device, the following problem is involved due to the fact that the image forming apparatus is loaded with a fixing unit. In the case where the tail end of the sheet P in the entering direction (namely, the lead end of the sheet P in the exiting direction) has a curled or warped portion, it is difficult to transport the sheet P having the curled portion properly into the downstream-located path 25 via the switchback device 7. A sheet jam or a folded corner end of the sheet P may likely occur when the sheet P is transported in and out of the switchback device 7.
Further, the sheet slapping operation in which the guide member 751 slaps the portion of the sheet P, which is certain distance away from the tail end thereof, against the tray 73 requires a fine adjustment of the timing to promptly transport the sheet P guided from the upstream-located path 24 toward the downstream-located path 25. The sheet transporting unit 74 is an essential element in the conventional switchback device 7 in association with the driving mechanism of moving the switchback roller 741 toward and away from the tray 73 in order to guide the sheet P in and out of the switchback device 7. The conventional switchback device 7 provided with these elements is required to have a complex structure and increases the production cost of the image forming apparatus due to the complex mechanism of accomplishing the sheet slapping operation in combination with driving the switchback roller 741 toward and away from the tray 73.
It is an object of the present invention to provide a switchback device that enables to accurately transport a sheet downstream with respect to a sheet transport direction while carrying out a switchback operation without a possibility of causing a sheet jam or a folded corner end of the sheet even if the sheet has a curled or warped portion.
According to an aspect of this invention, the switchback device focusing an image forming apparatus comprises: an inverting tray for temporarily holding a sheet guided inside the switchback device thereon; a guide unit for guiding the sheet in pressing contact with the inverting tray when the sheet is transported in an exiting direction on the inverting tray, the exiting direction being opposite to an entering direction of the sheet on the inverting tray; a guide switching unit for selectively changing the guide unit to a sheet guide allow state to allow the sheet to enter the switchback device in the entering direction on the inverting tray and a sheet pressing state to make substantially an end of the sheet in the exiting direction in pressing contact with the inverting tray; and a sheet transporting unit for transporting the sheet out of the switchback device in the exiting direction on the inverting tray.